1. Field of the Invention
The present invention relates to an optical system and more particularly, to an optical system of a projection exposure apparatus for transferring a specified mask pattern onto a substrate using a light source in the ultraviolet region for semiconductor manufacture.
Furthermore, the present invention relates to a method for manufacturing an image-focusing optical system which is suitable for use in the illumination optical system or projection optical system of a projection exposure apparatus.
2. Discussion of the Related Art
In the past, an apparatus such as the one shown in FIGS. 5A-5C has been used as a projection exposure apparatus for semiconductor manufacture. As shown in FIG. 5A, a light beam from a light source 51, such as a mercury arc lamp, is focused by an elliptical mirror 52, and is then converted into a parallel light beam by a collimating lens 53. As shown in FIG. 5(B) (or FIG. 5C), this parallel light beam then passes through a fly-eye lens 54 consisting of an aggregate of optical elements 54a with a square cross section, so that a plurality of light source images are formed on the emission side of this lens. An aperture stop 55 having a circular aperture is disposed in this light source image position. The light beams from the plurality of light source images are focused by a condenser lens 56, and are uniformly directed in superimposed form onto a mask M, which constitutes the object of irradiation.
The pattern on the mask M, which has thus been uniformly irradiated by the illumination optical system, is projected and exposed on the surface of a wafer W. Wafer W is coated with a resist by means of a projection optical system 57 consisting of a plurality of lenses 571 and 572. The wafer W is carried on a wafer stage WS, which moves in two dimensions. In the projection exposure apparatus as shown in FIG. 5, exposure is performed using a step-and-repeat scheme in which the wafer stage is successively moved in two dimensions stepwise to expose the next shot area, after the exposure of a given shot area on the surface of the wafer having a plurality of shut areas is completed.
In recent years, a scanning type exposure system has been proposed in which the pattern on the mask M is transferred onto the wafer W at a high throughput by irradiating the mask M with a rectangular or circular-arc-form light beam while synchronously scanning the mask M and the wafer W. Mask M and Wafer W are installed in conjugate positions with respect to the projection optical system.
In both types of projection exposure apparatus, it is desirable that the optical members used in the optical system have a high transmissivity with respect to the wavelength of the light source used. This is because the optical system of a projection exposure apparatus comprises numerous optical members. Even if the light loss per lens is small, the total effect of the drop in transmissivity, which is directly related to the number of optical members used, can be large.
Additionally, in the projection optical system, it is required that the uniformity in the refractive index of the optical members be high in order to obtain a finer and sharper projection exposure pattern. This is because propagation of light in the optical member depends on a local refractive index, and the image-focusing performance of the projection optical system is accordingly greatly affected by the variations in refractive index.
Accordingly, silica glass and calcium fluoride crystal, which have a high transmissivity in the ultraviolet region and possess superior homogeneity, are generally used as the materials for optical members employed in the optical systems of projection exposure apparatus operating in the ultraviolet region (at wavelengths of 400 nm or less).
In recent years, the shortening of light source wavelengths has been proposed as a means of transferring a fine mask pattern image onto the surface of a wafer with an increased resolution. For example, the shortening of wavelengths has progressed from the g-line (436 nm) and i-line (365 nm) used in the past, to KrF (248 nm) and ArF (193 nm) excimer lasers.
Thus, in projection exposure using such short-wavelength excimer lasers, a finer mask pattern image is obtained. However, as the photon energy of the light source itself is increased, so is the energy density of the light with which the optical members are irradiated. As a result of irradiation with light having a high energy density, the optical members are damaged. This damage leads to a drop in transmissivity and a deterioration in other optical characteristics, such as generation of fluorescence or increase in the refractive index.
In addition, to increase the throughput of a projection exposure apparatus, it is desirable to increase the number of substrates that can be processed per hour. It was believed that merely increasing the power of the light source may achieve this. However, increasing the illumination intensity has an adverse affect on the durability of the optical members. Thus, the recent demand towards higher throughput imposes more stringent requirements on the durability of the optical members.
Accordingly, the present invention is directed to an optical system for a projection exposure apparatus that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide an optical system for use in a projection exposure apparatus that has a superior durability under long-term irradiation with light.
Another object of the present invention is to provide a method for manufacturing such an optical system.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the present invention provides an optical system of a projection exposure apparatus for processing an excimer laser beam, the optical system including at least one optical member made of silica glass having a chlorine concentration of about 1 ppm or less, and at least one optical member made of silica glass having a chlorine concentration of about 1 ppm to about 200 ppm.
In another aspect, the present invention provides an optical system for use in a projection exposure apparatus for processing an excimer laser beam, the optical system including at least one optical member made of silica glass having a chlorine concentration of about 1 ppm or less, at least one optical member made of silica glass having a chlorine concentration of about 1 ppm to about 200 ppm, and at least one optical member made of calcium fluoride crystal.
In another aspect, the present invention provides a method for manufacturing an optical system for a projection exposure apparatus, the method including the steps of installing at least one optical member made of a first silica glass having a chlorine concentration of about 1 ppm or less, and installing at least one optical member made of a second silica glass having a chlorine concentration of about 1 ppm to about 200 ppm, wherein both optical members are installed in accordance with an energy density of light that the corresponding optical member is to process such that a drop in transmissivity caused by irradiation with the light becomes lower than that of an optical system using optical members made of only one of the first and second silica glass.
In another aspect, the present invention provides an optical system for a projection exposure apparatus including at least one optical member made of a first silica glass having a chlorine concentration of about 1 ppm or less, and at least one optical member made of a second silica glass having a chlorine concentration of about 1 ppm to about 200 ppm, wherein both optical members are installed in accordance with an energy density of light that the corresponding optical member is to process such that a drop in transmissivity caused by irradiation with the light becomes lower than that of an optical system using optical members made of only one of the first and second silica glass.
In another aspect, the present invention provides an optical system, including a light source for emitting a pulsed light beam, at least one first optical member for processing the pulsed light beam, the at least one first optical member being disposed at a location within the optical system where the energy density of the pulsed light beam exceeds a predetermined value, the at least one first optical member being made of silica glass having a chlorine concentration of about 1 ppm or less, and at least one second optical member for processing the pulsed light beam, the at least one second optical member being disposed at a location within the optical system where the energy density of the pulsed light beam is less than the predetermined value, the at least one second optical member being made of silica glass having a chlorine concentration of about 1 ppm to about 200 ppm.
In another aspect, the present invention provides an optical system, including a light source for emitting a pulsed light beam, at least one first optical member for processing the pulsed light beam, the at least one first optical member being disposed at a location within the optical system where the energy density of the pulsed light beam exceeds about 1 mJ/cm2/pulse, the at least one first optical member being made of calcium fluoride single crystal at least one second optical member for processing the pulsed light beam, the at least one second optical member being disposed at a location within the optical system where the energy density of the pulsed light beam is less than about 0.5 mJ/cm2/pulse, the at least one second optical member being made of silica glass having a chlorine concentration of about 1 ppm to about 200 ppm.
In a further aspect, the present invention provides an apparatus for projecting an image of a mask pattern on a mask onto a substrate, the apparatus including a light source for emitting a pulsed light beam an illumination optical system directing the pulsed light beam from the light source towards the mask, the illumination optical system including at least one first optical member for processing the pulsed light beam, the at least one first optical member being disposed at a location within the optical system where the energy density of the pulsed light beam exceeds a first predetermined value, the at least one first optical member being made of silica glass having a chlorine concentration of about 1 ppm or less, and at least one second optical member for processing the pulsed light beam, the at least one second optical member being disposed at a location within the optical system where the energy density of the pulsed light beam is less than the first predetermined value, the at least one second optical member being made of silica glass having a chlorine concentration of about 1 ppm to about 200 ppm, furthermore, a projection optical system directing the pulsed light beam that has interacted with the mask towards the substrate to project the image of the mask pattern onto the substrate, the projection optical system including at least one third optical member for processing the pulsed light beam, the at least one third optical member being disposed at a location within the optical system where the energy density of the pulsed light beam exceeds a second predetermined value that is smaller than the first predetermined value, the at least one third optical member being made of silica glass having a chlorine concentration of about 1 ppm or less, and at least one fourth optical member for processing the pulsed light beam, the at least one fourth optical member being disposed at a location within the optical system where the energy density of the pulsed light beam is less than the second predetermined value, the at least one fourth optical member being made of silica glass having a chlorine concentration of about 1 ppm to about 200 ppm.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.